Unity gain frequency (“fT”) of a MOSFET is an important radio-frequency parameter in analog circuit design. The unity gain frequency of a MOSFET is the frequency at which the amplifier's gain is equal to one. Oscillatory characteristics of a closed loop system begin at the unity gain frequency. Specific applications, such as envelope tracking systems, require precise control of fT variations in MOSFETs in a manufacturing line. If fT variations are not precisely controlled, the MOSFET will not oscillate properly, the gain will not be adequate, and the device capabilities will be affected.
Conventionally, radio-frequency statistics are generated using scattering parameter (“S-parameter”) measurements. Unfortunately, characterization time for S-parameters can be high, and tools for measuring S-parameters can be expensive. In a manufacturing line, this results in a reduction in output. Furthermore, S-parameter structures are large and must be placed in an area-constrained kerf, restricting space and productivity.